A fourth tree-sitter has joined three remaining protesters living in the branches of an oak grove outside Memorial Stadium, a university spokesman said today.

The man, whom university officials did not identify, climbed over one fence Sunday night and up a different tree from the one occupied by the other tree-sitters, UC Berkeley spokesman Dan Mogulof said.

Also Sunday, campus police arrested a 56-year-old supporter of the tree-sitters on suspicion of battery on a police officer and resisting arrest. David Walton was trying to get supplies to the tree-sitters when he got into an altercation with an officer and threw the officer to the ground, Mogulof said.

Last week, four of the tree-sitters left the trees, leaving three remaining protesters. The group is protesting the university's plans to cut down the oak grove to build an athletic training center.

The latest tree-sitter went up Sunday with a pack of supplies.

"It's his intention to stay as long as he can ... to save the trees," said Doug Buckwald, a spokesman for the protesters, who said the man is in his 30s.

Mogulof said the university is trying to coax all of the tree-sitters down. Officers have barred supporters from lifting supplies to the protesters, although the university is providing the tree-sitters with energy bars and water and is monitoring their well being.

Protesters have been in the trees since Dec. 1, 2006.

A judge is expected in the next several weeks to rule on whether the university can go ahead with plans to build the athletic training center next to Memorial Stadium. The City of Berkeley, the Panoramic Hill Association and oak tree supporters have sued to block the project.